1. Field of the Invention
The present invention relates generally to lubricant compensator systems for earth-boring bits. More particularly, the present invention relates to lubricant compensator systems for earth-boring bits of the shaft-boring variety.
2. Background Information
Earth-penetrating tools are divided into two broad categories, those designed to drill deep, relatively small diameter boreholes, and those designed to drill shallow, large diameter boreholes. Earth-boring bits with rolling cutters mounted on cantilevered bearing shafts often are called "rock bits" and are employed in drilling relatively small diameter boreholes for the recovery of petroleum or other hydrocarbons, or to tap geothermal energy sources. The nature of such drilling operations is that the operations are continued until bit life is nearly or completely expended. These rock bits, when used in mining operations such as the drilling of blast holes, generally are not recovered from the borehole until the bit is effectively destroyed. To extend the life of rock bits, many are provided with lubrication systems that include a pressure compensator to limit the pressure differential existing between the lubricant and the hydrostatic pressure in the borehole resulting from the presence of drilling fluid in the borehole.
In addition to pressure exerted on the bit by drilling fluids, temperatures increase in the lubricant as the bit is exposed to geothermal temperatures and to frictional heat build-up caused during rotation of the bit. These temperature increases cause an increase in the internal pressure of the lubricant, which causes the lubricant to expand. The increase in the internal pressure of the lubricant may also cause the lubricant to "crack" or emit gaseous hydrocarbons. If an earth-boring bit is of the type having positive seals between the cutters and bearing shafts on which the cutters rotate, the internal pressure build-up can be great enough to damage either the pressure compensator diaphragm or the seal between one of the cutters and its bearing shaft. A conventional practice in the rock bit field to avoid seal or diaphragm damage is to provide a pressure-relief valve in the lubricant reservoir. Such a pressure-relief valve permits release of lubricant upon the internal pressure of the lubricant exceeding a predetermined maximum.
Another type of earth-boring bit employs a plurality of rolling cutters, usually in excess of three, arranged to drill relatively large diameter boreholes for mining applications. These bits are used for shaft boring, which results in large-diameter boreholes or shafts. In shaft boring operations, the bit is secured directly to a drilling machine and is rotated and pushed through formation material to bore a shaft. Drilling fluid commonly is used and exerts hydrostatic pressure on the bit, while frictional heat build-up and geothermal temperatures increase the temperature in the lubricant.
Unlike rock bits, it is not a particularly common practice to provide lubricant compensation systems in shaft-boring bits. This is because shaft-boring bits are recovered from shallow shafts with substantial operational life remaining. After each recovery, lubricant can be replenished and bearings and seals repaired or replaced easily. However, these routine maintenance operations are inconvenient and increase expense. Additionally, in drilling deep or long shafts, the operational life of the shaft-boring bit may be expended before recovery, rendering it advantageous to extend that life as much as possible.
U.S. Pat. No. 3,419,093, Dec. 31, 1968 to Lichte et al. discloses a cutter assembly for a shaft-boring bit in which the cutter assembly is provided with a longitudinal bore. A lubricant reservoir is defined in the journal of the cutter circumferentially surrounding the bore. An annular flexible member encircles the longitudinal bore and serves to enclose the lubricant reservoir formed in the journal. The annular flexible member is exposed to hydrostatic pressure through the longitudinal bore during drilling operation and thus serves as a lubricant compensator. However, the displacement of the annular member is limited, thus limiting its ability to compensate for pressure imbalances, and no pressure-relief mechanism is provided.
Commonly assigned U.S. patent application Ser. No. 08/137,651, now U.S. Pat. No. 5,363,930, Nov. 15, 1994, to Hern discloses a lubricant compensator system for raise- or- shaft-boring bits that employs two resilient diaphragms, one of which is in fluid communication with the exterior of the bit and compensates for hydrostatic pressure. The other diaphragm compensates for increases in internal pressure of the lubricant.
U.S. Pat. No. 4,597,455, Jul. 1, 1986 to Walters et al. discloses a rock bit lubrication system in which lubricant compensator assemblies are provided in the cantilevered bearing shaft of a rolling cutter rock bit having three cutters. Provision of the compensator assembly in the bearing shaft of a rock bit is not practical due to the dimensional constraints of the bearing shaft because the lubricant reservoir is reduced in volume and the removal of material from the bearing shaft required to mount the lubricant compensator assembly can drastically weaken the bearing shaft, leading to premature bit failure.
A need exists, therefore, for a shaft-boring bit having cutters with lubricant compensators that make efficient use of space within the cutter assembly and that are provided with a pressure-relief means to avoid damage to cutter seal member resulting from internal pressure build-up in the lubricant.